Positive feedback pulse generator



J. w. BALDE'ETAL 2,657,307

POSITIVE FEEDBACK PULSE GENERATOR Oct. 27, 1953 Original Filed 001;. 6, 1948 6 Sheets-Sheet 2 INVENTORS M ATTORNEY Oct. 27, 1953 J. w. BALDE Er n.

POSITIVE FEEDBACK PULSE GENERATOR 6 Sheets-Sheet 5 Original Filed Oct. 6, 1948 INVENTORS Jasep liflryar Kmeil. m A) ATTORNEY Oct. 27, 1953 JQw; BALDE ETAL POSITIVE FEEDBACK PULSE GENERATOR Original F iled Oct. 6, 1948 e Sheets-Sheet 4 INVENTORQ Q fim Wjalde J'ase v agar mneZfi /L'. 'kaymzw, [MW ATTORNEY NNN Oct. 27, 1953 J. w. BALDE EIAL POSITIVE FEEDBACK PULSE GENERATOR Original Filed Oct. 6, 1948 6 Sheets-Sheet 5 m 0 RN m e \\N\ W W N m a NE 3 In F w Qw EN MEN N T M YEN @w M l A, ATTORNEY Oct. 27, 1953 J. w. BALDE ETAL POSITIVE FEEDBACK PULSE GENERATOR 6 Sheets-Sheet 6 Original Filed Oct. 6, 1948 P atented Oct. 27, 1953 UNlTED STATS PTENT OFFICE POSITIVE FEEDBACK PULSE GENERATOR Original application October 6, 1948, Serial No.

53,076, now Patent No. 2,626,980, dated January 27, 1953. Divided and this application July 1, 1950, Serial No. 171,732

8 Claims. 1

This invention relates to a marking pulse generator and more particularly to a positive feedback pulse generator wherein a plurality of crystals with different resonant frequencies provide input voltage pips to a plurality of demodulating and amplifying tubes which clip and mix the resulting amplified signals to form a composite marking pulse having separate traces representative of each of the different resonant frequencies.

This application is a division of a copending application, Serial No. 53,076, filed October 6, 1948, now Patent No. 2,626,980, dated January 2'7, 1953, wherein a control system for testing the frequency response of tuned devices is described and claimed.

In the past, curve tracers have been utilized to present frequency response Wave forms so that a device under test may be tuned to maximum response. This was accomplished by coordinating the horizontal sweep of an oscilloscope with the changes in frequency applied to the device under test in such a manner that the resulting voltage wave form appeared as a plot of the output fre quency response on the vertical axis against frequency n the horizontal axis. However, this method yields only qualitative results. A later method applied two standard frequency response curves to the screen of an oscilloscope for use as go, no-go limits for a third frequenc response curve which was derived from a device to be tested. However, this method required such a large number of standard resonant devices that it was not suitable for production line usage.

Accordingly, it is one of the objects of this invention to provide an electronic curve tracer with indicia of frequency which is synchronized with the frequency variations applied to a device under test so that accurate quantitative measurements can be quickly made upon electrical devices and which is also suitable for production line usage.

With this and other objects in view, the invention comprises a curve tracer for testing the resonant qualities of an electrical device wherein a frequency modulator applies a frequency modulated voltage to a device to be tested for generating a frequency response voltage which is to be compared on the screen of a cathode ray tube with a plurality of constant bandwidth frequency markers produced by a plurality of crystal controlled positive feedback pulse generators which are energized by the frequency modulated voltage. The curve tracer also includes a plurality of attenuators responsive to a variation inthe input voltage of the device under test for applying a plurality of definite decibel (db) level attenuation limits to the screen of the cathode ray tube.

Other objects and advantages of this invention will more fully appear from the following detailed description, taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic block diagram of the circult embodying the present invention Figs. 2a-2h comprise a plurality of curve tracer wave forms showing various stages of filter tuning obtained when a filter is tested in the apparatus of this invention;

Figs. 3aJ-3b show the manner in which subsequent Figures 5 to 8 of the drawings are positioned adjacent each other to show a specific embodiment of the invention;

Figs. la-4c show curve tracer wave forms of the frequency response of a plurality of devices tested in the present system to illustrate the flexibility thereof; and

Figs. 5 to 8, when arranged as shown in Figs. 30-31;, show in detail circuit arrangements of certain elements of a typical system incorporating the present invention.

Referring now to Fig. l, a saw-tooth wave oscillator 20 is provided for modulating the output from. a frequency modulation oscillator 2| so as to produce a frequencymodulated signal. The frequency of. saw-tooth wave oscillator 20 is controlled by a synchronizing signal from multivibrator 39, thereby causing oscillator 20 and multivibra-tor 30 to operate at the same frequency. Multivibrator 30 frequency is held at a multiple of the cycles per second power line frequency by means of a synchronizing pulse generator 3i. The saw-tooth modulating signal is adjusted to provide a frequency modulated signal with a band sweep of any desired frequency width such as '75 K0. in addition, the saw-tooth wave oscillator 20 also provides a sweep voltage which is applied to the horizontal deflection means of a cathode ray tube 22.

The output from the frequency modulation oscillator 2! is applied to a circuit 23 whose function is to utilize the frequency modulated signal to generate a pair of frequency bandwidth markers which are a certain adjustable bandwidth apart. The markers thus generated are applied to the intensitymodulation means of the oathode ray tube '22. As the frequency marker circuit is energized by the frequency modulated signal, the drift in the oscillator 2 is of no consequence since the drift in the oscillator 21 pro- Ill duces a change in output frequency and scale frequencies alike so that the scale produced by the markers remains unchanged in absolute frequency bandwidth.

A continuous wave oscillator 24 of an adjustable constant frequency output is connected to a mixer tube 25 to provide a heterodyning frequency signal which is mixed with the frequency modulated signal from the oscillator 2! to provide an output frequency modulated voltage with an adjustable mean frequency. By temperature compensating the oscillator 26, it is possible to derive a test voltage from' the mixer tube 25 which has a frequency bandwidth of a constant mean frequency. Thus by varying the output frequency of the oscillator 24, it is possible to vary the frequency range over which the test device frequency characteristic is obtained.

The frequency modulated signal of the desired mean frequency is applied to a circuit 25 comprising a band pass filter and a limiter circuit which serve to remove the extraneous frequency voltages and to provide a frequency modulated voltage of constant amplitude. It is essential that the test voltage be of constant amplitude in order that the visual indication of the frequency characteristic of a device 21 to be tested is presented in an undistorted form which is suitable for measurement by a direct reading on the cathode ray tube screen. 7

This constant amplitude voltage is applied to the test device 21 in order to generate a frequency response voltage which is proportional to the frequency characteristic of the device 21 to be tested. A plurality of attenuators 28 is also connected to the same source of voltage as that which is applied to the device 21 so as to produce a plurality of attenuation (voltages which are at definite db levels with respect to the input voltage applied to the device 21. This method of energizing the attenuation means assures an accurate Thevenin voltage measurement on the screen of the cathode ray tube 2 2 inasmuch as any change in the input voltage of the device 21 causes a corresponding change in the level of the attenuation lines. When the internal impedance of the voltage source is of the same magnitude as the input impedance of the device 21,.

the variation of input impedance caused by the varying applied test frequencies produces a corresponding change in the input voltage and subsequently in the attenuation lines. However, when the input impedance of the device 21 is larger than the internal impedance of the voltage source, the variation of input impedance does not affect the input voltage and the attenuation lines remain unchanged.

An electronic switch 29, which is triggered by a multivibrator 3% is connected to the test de.. vice Z1 and'to the attenuators 28 'so that the frequency response voltage and the attenuator voltages are passed in sequence through detector: 32 which eliminates the high frequency com-" ponents. The demodulated frequency response voltages are amplified by amplifier 33 and then impressed in sequence on the vertical deflection means of the cathode ray tube 22. The multivibrator 30 is adjusted to trigger the'switch 29 so that one complete frequency response or attenuation voltage wave form is applied to the verticaldefiection means for each complete horizontal sweep of the electron beam. In this manner, the speed of the sweeping voltage and the 4 tested superimposed upon the traces of the attenuation lines and the frequency markers. The simultaneous comparison of the frequency characteristic curve with the frequency and attenuation limits allows rapid adjustment and align- .ment of such test articles as filters and radio frequency receivers during the period in which the test is being performed.

In Fig. 5, a saw-tooth oscillator tube 4| is shown having a grid 40 connected to an input jack 12 through a resistor 43 and a variable resistor 44; thus a synchronizing voltage may be applied to the grid 40 to control the frequency of oscillation of the tube 4!. A condenser 45, connected across the tube 6!, is charged by the voltage from a B+ supply during the period which the tube a! is not conducting. When a positive pulse of voltage is placed on the grid 40 from the input jack 42, the tube 41 conducts, thus allowing the condenser 45 to discharge quickly. The charging and the discharging of the condenser 45 produces a voltage of a substantially saw-tooth wave form which is applied to an output jack 46 through a condenser 41. This saw-tooth wave is applied from the jack 5% to the horizontal deflection means of the cathode ray tube 22 to provide a linear sweeping voltage. a This saw-tooth voltage is also applied to a control grid @9 of a reactance tube 56 through the condenser l: and a variable resistor at. A plate (it of the tube is connected to a control grid 6! through a condenser 62, a resistor 63 and a resistor 6 A phase shifting network comprising the condenser 82 and the resistors 63 and 64 serves to place a voltage on the control grid S! which is 90 out of phase with the voltage on the plate fit of the tube 58. Since the plate 543 is supplied with voltage from a tuned circuit 65 of a Colpitts oscillator tube 66 through a condenser 61, the voltage of the control grid 5! is 90 out of phase with the voltage existing across the tuned circuit 65. The tuned circuit 65 includes an inductance coil 1! and a pair of condensers 12 and 13 which are shunted in parallel between a platelfi and a control grid 15 of the'tube 65. Because of the 90 phase shift in the voltage on the control grid Si, the current flowing through the tube 50 is 90 out of phase with the voltage existing across the tuned circuit 65 so that the tube 5!] acts as a reactance in its effect on the tuned circuit $5. The reactance effect of the tube 56 causes a variation in the frequency of oscillation of the tube 66 by an amount which is proportional to the instantaneous direct current value of the saw-tooth wave impressed on the control grid as of the tube 50. The impressed saw-tooth modulating voltage on the grid t9 Varies the plate current of the tube with the result that the actual magnitude of the reactance effect of the tube as is varied in accordance with the plate current of tube 50, thus causing a variation in the oscillator tube frequency output persistence of the screen of the cathode ray tube V 22 coact to present a single picture of the frequency characteristic curve of the deviceIZl-to be from the tuned circuit 55 due to the resulting change in the LC ratio of the oscillator tank circuit 55. This variation in the reactance effect of'the tube 50 determines the maximum frequency band sweep of the frequency modulated voltage of the oscillator tube 66. The frequency modulated voltage from the tuned circuit is then applied to a control grid 68 of a mixer tube E59 by means of a condenser 10.

A continuous wave oscillator tube is adjustedto a certain frequency of oscillation by the selection of the proper size of the adjustable circuit elements which comprise the tuned tank circuit of the Colpitts oscillator tube Bil. This aerator 5. tuned circuit includes a plurality of adjustable inductance coils- 8],- a pair of temperature com pensa-ted condensers 8-2 and 83, andavariable condenser 84" which are all shun-tedin; parallel between a plate 35' and a control grid 86- of the tube 8E. Ihe frequency of oscillationof the tube 89 is adjusted by the choice of the inductance coils 8! while the temperature compensated condensers 82 and t3 serve to prevent any fre-- quency drift of the oscillator tube- 89- due totemperature changes. The constant frequency output from the tube 39' is coupled to a control grid 53? or the mixer tube 59 by acond'enser 83.

The frequency modulated voltageapplied to the grid 55 is heterodyned intube (59 by the con-' stant frequency voltage applied to the grid 81 so as to provide a carrierfrequency of onedesiredvalue which is frequency modulated over a periodically varyingpreassigne'd range. The mean frequency of the voltage obtained from the tube to may be converted to any desired different mean frequency by achange in the value of the inductance coil 84, which change varies the frequency of the heterodyning voltage from the tube 8t; This method of changing the mean frequency of the frequency modulated wave prevents any change in the bandwidth of the frequency modulated wave.

The frequen y modulated voltage from a plate a! of the tube so is applied to a control grid 32 of a cathode follower tube 9-3 through a condenser and a conductor st. The voltage applied to the grid at varies the current passing through the tube 93 so that a voltage is producedon a cathode resistor HI in accordance with the variation of the plate current in the tube 333. The cathode follower tube 93 provides a means of matching a relatively low impedance band pass filter H2, connected to the resistor III by a coupling condenser H3 and a conductor- H i, to the relatively high impedance voltage source. ihe band pass filter H2 removes any undesired high frequency voltages which may be present in the circuit from the oscillator tubes 56 and 88.

The filtered output from the filter [i2 is impressed on a grid IE of a triode amplifier tube Isl through a conductor 532. The amplified frequency modulated voltage from the tube ISI is coupled to a grid H33 of a triode amplifier tube I3 3 through a coupling condenser 535. V The amplified voltage from the tube I34 is applied through a coupling condenser I545 to a gridvoltage limiter circuit comprising a rectifier It! and a battery I52 which are connected in parallel with a resistor fist. When the voltage output from the tube i3 exceeds the Value of the volt-' age of the battery 152, the rectifier I'M applies the excessive voltage amplitude to ground so that no voltage pulses over a certainpredetermined amplitude are applied to a grid I58 of a cathode follower tube its through a conductor ill. his a result of the action of the limiter cir'-' cult, the peak of the voltage wave applied to the control grid E5? is clipped oil in a manner as illustrated by the Wave form shown on the draW= ing in conjunction with the grid lead I'll. By limiting the amplitude of the voltage applied to the grid 55 5, the frequency modulated voltage output of the tube 555 is limited to a constant amplitude.

The variation in the voltage applied to the grid I 55 varies the plate current passing through a cathode resistor El? and the tube I55 so as to produce a varying voltage on the highpotential end of the resistor in; The constant amplitude frequency modulatedvoltage' from the resistor I l! is connected to an output jack I-'I 3' through aconductor I 14.

The" fre uency modulated voltage generated by the oscillator tube 5 3- is also applied to a control grid 89 of a triode amplifier tube 911 through a conductor I 69-; The frequency modulateuvoltage; is ampuned in the tube 91] alf'ld then coupled to agrid I-ll-i of a cathode follower tube 92- tl irough a coupling condenser F03 which is connected toa" plate Hi l of the amplifier tube The voltage applied to the grid ill-I- varies the plate current flowing through the tube [92 so that a?- vai ying voltage is roducedat the high potential end of a cathode resistor I85.

The varying voltage produced by the resistor N35 is coupled to: a pair or crystal filter circuits m6 and is? through a pair of cou ling condensers Its-end IE9. The crystal filter It? comprises a plurality of crystals Hit and avariable condenser I20 which are connected in parallel. The crystal filter I02: comprises aplurality of crystals I 21 and a variable condenser 22 which are connected in parallel.- The frequency modulated signal is applied to the filter circuit is? through the coupling. condenser Ills to produce a voltage pip at a point in the frequency spectrum which is determined by the series resonant frequency of the crystal Hi]; The variable condensers I'ZI! and I22 neutralize the capacitance of thecrystals He and lei, respectivel so as to' suppress spurious responses of the crystal filters I38 and i-ii'l. The frequency at which the voltagepip is generated may be varied by changing a switching means I23 so that a crystal- Hi! which is cut to resonate at a different frequency may be placed in the filter circuit 58?. The crystal filter I86, in cluding a switching means I36, functions inthe same manner as the circuit til-I and consequently is not explained in detail The difference in frequency at which circuits Hi6 and It? generate their frequency disturbance is the bandwidth which is intercepted between the frequency markers on the screen of cathode ray tube 22. For convenience, the crystal filter circuit I B? is assumed to generate a disturbance at a lower frequency than the crystal filter circuit its" although such an assumption is not essential to the operation of the marker system.

Referring now to Fig. 6, a conductor I3! isconnected to the crystal filter circuit It? and to a controlgrid #24 of a triode amplifier tube I25. The voltage disturbance produced by the crystal is amplified by the tube :25 and then impressed on a grid I26 01"- a clipper amplifier tube I2? through a coupling condenser I28. A rectifier 23, which is connected from the grid I26 of the tube I2? to ground provides detection of the high frequency component of the applied voltage and, therefore, serves to accentuate the variation in the envelope of the applied voltage wave form.

The Voltage output from the tube I2! is coupled by a coupling condenser to a clip per positive feedback circuit comprising a rectifier I'M and a" variable resistor I 42 which are connected in parallel between a plate I43 of the tube I21 and ground. The rectifier l iI serves to conduct any positive pulses to ground so that an entirely negative intensity marker pip is produced by the amplification of the frequency disturbancezfrom the circuit I01. The voltage thus limited by the rectifier MI is applied across the feedback resistor I 42. An adjustable amount ofthevoltage developed across the resistor I42 is applied to the grid I24 of the first amplifier tube I25 through a resistor I44 and a conductor I45. This feedback voltage reinforces the envelope variation of the frequency disturbance originally impressed on the grid I24 from the circuit 101 so that the combination of the tubes I25 and I21 with their associated circuits acts essentially as a wave shaper or pulse generator which generates a clearly defined marking pip from the frequency disturbance produced by the crystal filter circuit I01. The marking pip produced by the tubes I25 and I21 is impressed on a cathode I46 ofa mixer tube I41 through a coupling condenser I48; A cathode load and bias resistor Ht connects the cathode I46 to ground and by-pass condenser II1 connecting one terminal of the condenser I48 to ground conducts any undesired transient voltages to ground.

The higher frequency disturbance produced by the crystal filter circuit I06 is connected to a grid 549 of a triode amplifier tube I50 by means of a conductor IE6. The voltage impressed on the grid M39 is amplified by the tube I50 and coupled to a grid lei of a clipper amplifier tube I62 by means of a coupling condenser I63. Inasmuch as the tubes I59 and I82 together with their associated clipper circuits comprising a pair of rectifiers I53 and IE5, and their associated feedback circuit including a variable resistor I66, a

fixed resistor I61, a conductor I68, and a coupling condenser I51 function in the same manner as their identical counterparts explained in conjunction with the tubes I25 and I21, the operation of the present circuits are not explained in detail. The frequency marking pip generated by the combination of the tubes IE and I62 is impressed on a cathode I59 of the mixer tube I41 through a coupling condenser I and a conductor I89. A cathode load and bias resistor IE8 connects the cathode N59 to ground and a by-pass condenser H9 connecting one terminal of the condenser I13 to ground conducts any undesired transient voltages to ground.

Since grids I8! and I22 of the mixer tube I41 are connected to ground, the pulses applied to the cathodes MS and I69 vary the plate to cathode voltage of the tube I41 so that plate current fiows from the cathodes I46 and IE9 to a pair of plates I83 and I84 in accordance with the positive pulses which are applied to the oathing may be varied to any desired bandwidth by switching difierent crystals III] and I2] into the circuit. A resistor I89 and a rectifier I90 which are connected in series between the plates I83, Ed -i and ground serve to clip the unwanted positive portion of the amplified frequency marker pips.

fied by the tube 486 and then coupled through a conductor 2535i and a condenser I58to a grid 253i of a clipper tube 262 which is biased so that only the large positive pulses of voltage of the frequency markers cause a change in the plate current fiowing through the tube 202. By biasing the tube 292 to this point of operation; all

The voltage applied to the grid I65 is ampli of the low voltage base line disturbances are removed from the voltage output. This voltage is coupled to an output jack 203 through a coupling condenser 204. A typical marker wave form of negative intensity is illustrated on the drawing in conjunction with the output jack 283. The output voltage is in turn impressed upon the intensity modulation means of a cathode ray tube 22.

A power input plug 204 is provided with four terminals, one of which, a terminal 205, is connected to ground. A terminal 206 supplies a positive rectified voltage to a plurality of dropping resistors 201 through a conductor 288. The resistors 201 are connected in parallel between the conductor 208 and a plate 209 of a gaseous voltage regulator tube 2IU. The resistors reduce the voltage applied from the terminal 206 to a certain lower value, for instance volts, and the regulator tube 2IIl maintains this voltage at a constant value by conducting excess voltage to ground through a conductor 220. A filter condenser 22I is connected in parallel with the tube 2I9 to by-pass any undesired high frequency voltage to ground. A conductor 222 applies the reduced voltage to the plurality of elements in the circuit while a conductor 223 applies the higher voltage B supply from the terminal 206 to the plurality of circuit elements. A primary winding 224;. of a filament transformer 225 is connected between a pair of terminals 226 and 221 of the input jack 284. A secondary winding 228 of the'transiormer 225 has two output terminals a:--:c which are connected in parallel with all the cathode heaters of the tubes in the circuit to provide a source of filament voltage.

Referring now to big. '1, an input plug 229 is provided with a plurality of terminals, one of which, a terminal 230, is connected to ground. A terminal 225i is connected by a conductor 232 to a plurality of elements in the circuit for the purpose of supplying positive B voltage of approximately 300 volts in magnitude. A primary winding 233 of a filament transformer 22% is connected across a pair of terminals 235 and 2 38 which are energized by standard A. C. supply voltage. A secondary winding 24! of the transformer 234 terminates in a pair of leads y-y which are con-' nected in parallel with all of the designated cathode heatersof the tubes found in the circuit comprising Fig. 7 and Fig. 8. A primary winding 2 32 of, a high voltage transformer 2 13 is connected across the terminals 235 and 249 of the input jack 229 to supply A. C. voltage to a secondary winding 24d of the transformer One terminal of the windingZd lis connected to a plate 245 and a cathode 226 of a rectifier tube 2 31. The other terminal of the winding 246 is connected to a plate 248 through a condenser 2 5i! and to a cathode 250 through a condenser This manner of connection provides a standard voltage doubling rectifier circuit, the output of which is appliedto a voltage regulator tube 262 through a dropping resistor 25!. A filter condenser 263 is shunted between a plate 264 and a cathode 26".? of the'tube 252 so as to provide a by-pass for any stray alternating current components of voltage.

Since the voltage of positive polarity applied to the plate 264 of the tube 262 is connected to ground by a conductor 26%, the tube 252 provides a constant, regulated voltage of negative polarity to a conductor 281. The conductor 2S1 supplies the negative bias voltage to a plurality of control grids in a ring counter system 2'58.

A time delay relay 21! including a contact 212 9 and a connection arm 2'13 is connected between the A, C. terminals 235, 2330i the input plug 229 tov provide a means for retarding the application of the negative bias to the grids of the tubes in the ring counter system 268. The time delay allows the ring counter system 253 to adjust to proper initial operating condition before all tubes y scribed circuit including the tubes 365 and 303 is a conventional Eccles-Jordan flip-flop circuit are biased into an operative condition. A manual 2 switch 2'14 connecting two portions of the conductor 267 is provided for determining the number of paths of the ring counter system 263 which are to be energized. When the switch 2755 is connect ed to a pair of contacts 275, the grid bias voltage is removed from all but one of the counter system stages so that only one path is conducting. When the switch. 214 connects to a pair of normal operation contacts 216, all of the stages of the counter system are energized. A pair of lamps 271 and 218 which are connected in parallel with the, secondary winding 213i give a visual indication of the position of the switch 214..

The ring counter system 263 is triggered from the output from a conventional asymmetrical multivibrator tube 2-39. A plate 21% of the tube 269. is connected to a grid 233 through a condenser 28I. Another plate 232 of the tube 239 is connected to a grid 283 through a condenser 283. An adjustable grid resistor 285 is connected from the. grid 2.30v to. ground and another grid resistor 286 is connected from the grid 283 to ground. This conventional multivibrator arrangement produces a pulsating output voltage, the frequency of which isv controlled by the RC time constant of the two resistor condenser combinations which include the resistor 235 and the condenser 28 l and the resistor 28.6 and the condenser 284. In order to prevent any frequency drift, a synchronizing voltage is applied to the grid 233 of the tube 269 from a line frequency synchronizing tube 28 through a conductor 283 and a coupling condenser 289.

'A grid 29!) of the tube 28! is connected to one of the cathode heater connections 1 through a condenser 28| and also to. ground through a rectifier 3110. The 60. cycle voltage from the cathode heater isrectified by the rectifier 333 so that only the positive pulses of the alternating current voltage are applied to the grid 2%. This applied voltage causesthe tube 237. to emit a larger plate current during the application of the positive 60 cycle pulse so that the voltage coupled to the grid 283 of the tube 289 through the conductor 238 consists of an irregular voltage wave of 60 cycle frequency. When the frequency of oscillation of the tube 269 is. adjusted to some value of frequency which is a multiple or subernultiple or" the injected 60. cycle voltage, the voltage applied to the grid 283 serves to hold the multivibratcr output frequency in synchronism' with the control voltage which in this instance is the 60 cycle line voltage frequency. As disclosed hereinbefore, this-synchronization prevents. any 60 cycle motion on the screen of the cathode ray tube 22..

The output voltage from the multivibrator tube 239 is applied to an output jack 3!!! through a conductor 3G2 and a coupling condenser 33-3. From the jack 3131. the voltage is supplied to the input jack 42 of Fig. 5 so that the saw-tooth wave oscillator 41 operates. in synchronism with the switching rate of the tube 253.

The output voltage from the tube 253 is also applied to a grid 3.9.4 of a tube 3B5 through a coupling condenser 306.. The. grid 33 i is connected to a plate 3.6.! or a tube 308 through a fixed resistor 309. A. grid 3 l 9 of the. tube 3.33,.is connected elements of the circuit are not described.

with two stable conditions.

Assuming that the tube 365 is conducting and that the tube 398 is non-conducting, the application of a negative pulse from the multivibrator on the grid 304 of the tube 305 causes a reduction in. the plate current which produces an increase in the voltage on the plate 32.3. The increased voltage as applied to the grid 3l0 of the tube 338 with the result that the tube 308 emits a plate current which produces a decreased volt-- age on the plate 301. The decreased voltage is applied to the grid 304 through the resistor 39 so that the bias on the. grid, 304 causes the tube 335 to become non-conducting. The emission 'of the plate current in the tube 338 also produces an increased voltage across a cathode re,- sistor 32 3 which is connected between a cathode and ground.

Inasmuch as a plurality of pairs of tubes, 326 and 32?, 328 and 329, 331] and 33, are interconnected. in. the same conventional Eccles-Jordan flip-flop system as the tubes 3% and 3138, the In order to simplify the explanation of the operation of the ring counter system 238,v it is also assumed that at the time when the tube 335 is conducting and the tube 303 is non-conducting, that the tubes 32.7,. 329, and 340'are conducting and that the. tubes 326., 328, and 333 are non-conducting.

When the tube 335 ceases to conduct as hereinbefore explained, a positive pulse is coupled to a grid 33! of the tube 326 through a coupling condenser 333. This positive pulse opposes the negative pulse from the multivibrator 269 which is applied to the grid 34! through a coupling condenser 332 and the conductor 3.32. The rise in voltage on the grid 3M causes a decrease in the plate current so that the tubes 32.6 and 32'! function in the same manner as hereinbefore described in conjunction with the tubes 305 and 308. The remaining pairs of tubes 32.8 and 329, and 330 and 346 are also triggered in sequence and placed in operation in the same manner. The positive voltage pulse from a plate 344 of the tube 33!! is conducted to the grid 304 of the tube 395 through a conductor 3.45 and a coupling, condenser 346. In view of the. hereinbefore described method of operation of. the ring counter system 268, it is seen that. th tubes 333,. 321., 3.2.9, andv 3.40 conduct in. a predetermined sequence and that fol.- lowing conduction, the tubes cease conducting until triggered again by the preceding flip-flop circuit. This. sequence of conduction produces a voltage wave form at each of. a plurality of oathode resistors, 3.24, 347, 348, and 3 39, which is composed of. one pulse of low voltage representing the period during which there is no current flow and three pulses of high voltage magnitude which represent the period during. which the. associated tube conducts. The four cathode voltage wave. forms. difier in the. position of the low voltage pulse in. accordance with the position of the tube in the conduction sequence. Four typical wave forms are shown in Fig, 6 in conjunction with the four cathode resistors 324, 341, 348, and 349.

The voltages, derived from. the cathode resistors 32.3, 3 2?, 343, and. 349 are applied to a plurality of cathodes sec, 339, as: and 362 of a plurality of gate amplifier tubes 393, 394, 365, and 395 through a plurality of conductors 351, 3'89, and 3'59. Inasmuch as the cathodes 359, 35 .1, 35!, 352 share the cathode resistors 324, 34?, 34B, 349 with the ring counter tube 398, 32?, 329, 349, the biasing voltage applied to said cathodes will be the same voltage as that which is developed across the cathode resistors 324, 341, 348 and 349. Therefore, the low voltage pulses disclosed previously as existing during the period when the tubes 353, 321, 329, 349 are non-conducting are produced by the normal flow of plate current through the tubes 363, 354, 395, 355 and their associated cathode resistors 324, 341, 348, 349. When the high voltage pulses are applied to the cathodes 359, 339, 33!, 332, the tubes 3'63, 394, 355, 3% do not conduct as the plate to cathode voltage is too small. The application of the low voltage pulse to the cathode increases the plate to cathode voltage so that the tube conducts. Therefore, the tubes 333, 394, 335, and 355 conduct only in the predetermined sequence in which the low voltage pulses are applied to the cathodes 359, 363, 33! and 362.

Referring now to Fig. 8, a grid 399 of a cathode follower tube 38! is connected through a conductor 382 and a variable resistor 383 to an input jack 384. The jack 384 is connected to the jack I73 of Fig. 5 so that the frequency modulated voltage from the tube !55 is impressed upon the grid 399 to vary the flow of plate current through the tube 38! and an associated pair of serially connected cathode resistors 385 and 395. The varying voltage impressed on the grid 38!! varies the plate current flow so that a varying voltage is provided at the high potential end of the resistor 385. This varying voltage is applied to a device 2'2 to be tested through a conductor 387 which is preferably either shielded or a coaxial cable and an output jack 338. The cathode follower tube 38! serves to match the impedance of the voltage supply circuit to the low impedance of the device 27 to be tested. Through the impedance matching, it is possible to use long connecting lines to place the test device a great distance from the rack mounted electronic circuit without excessive pickup due to the long line. It also makes it possible to use the same equipment to test a wide range of devices of varying impedance through the expedient of changing the impedance match of the cathode follower tube 33!.

The output frequency response voltage derived from the device 21 to be tested is applied to a grid 389 of a frequency compensated amplifier tube 399 through a conductor 495 and an input jack 39 The tube 399 amplifies the voltage impressed on the grid 389 without phase distortion so that a true wave form of the frequency response voltage will be presented on the screen of the cathode ray tube 22. The amplified response voltage is coupled to an output jack 4!)! through the coupling condenser 492 and an attenuator 392.

A grid 493 of a frequency compensated amplifier tube 494 is connected to the input of the device 2! through a conductor 435. The voltage impressed on the grid 493 is amplified and then applied to a grid 495 of a frequency compensated amplifier tube 431, through a coupling condenser 498. The amplified voltage from the tube 49! is connected to a plurality of adjustable resistors 439, 4 l! and 429 through a coupling condenser 42! and a conductor 422. The variable resistor 409 is connected to a grid 423 of a cathode follower tube 424 through a conductor 425., The resis o 499 is set to a certain value of desired attenuation and the resultant voltage developed across the resistor is coupled to the grid 423 so as to allow a certain amount of plate current to flow through the tube 424 and a pair of cathode resistors 426 and 521. The voltage variation from the high potential end of the resistor 42'! is conducted to an attenuator 393 through a conductor 429. In a similar manner, th resistors M9 and 429, conductors 439 and 43!, grids 449 and 44! of a pair of cathode follower tubes 442 and 443, cathode resistors 445, 446, and 447 conductors 448 and 449 serve to provide a pair of voltages to a pair of attenuators 394 and 395. By varying the adjustable resistors 499, M0 and 42 9, it is possible to compensate for unequal losses in the tubes 424, 442 and 443 so that the same values of voltage will be coupled to the attenuators 393, 394, and 395. The output voltages from the attenuators 393, 394 and 395 are applied to a plurality of output jacks 423, 459 and 459 through a plurality of conductors 45!, 452 and 453 which preferably are shielded conductors or coaxial cables. These output voltages are adjusted to the different attenuation levels desired on the screen of the cathode ray tube 22 by adjusting the attenuators 393, 394 and 395. A voltage regulator tube 46! and a condenser 492 are shunted across the tubes 38! and 424 to stabilize the voltage applied across said tubes. A similar voltage regulator tube 453 and a condenser 454 are also shunted across the tubes 442 and 443 in order to stabilize the voltage. The number of attenuation levels provided by the plurality of attenuators 393, 394 and 395 is not essential to the inventive principles of th circuit and a greater or lesser desired number of levels may be provided by varying the number of the resistors and their associated cathode follower circuits.

The hereinbefore described method for providing attenuation voltage allows Thevenin voltage measurements to be made on the screen of the cathode raye tube 22. Inasmuch as the attenuation resistors 393, 394 and 395 are energized from the input of the device 2'! to be tested, the input impedance pull-down voltage is equalized by a corresponding variation in the plurality of at tenuation voltages so that the attenuation lines move in accordance with the changing input voltage of the device 2'! to be tested. If the attenuation lines are energized from a separate voltage source, the drop in the input voltage to the device 2'! produced by the varying input impedance introduces an error in measurement when the decreased frequency response voltage is measured by comparison with the fixed attenuation lines. The effect of the varying input impedance of the device 2! may be seen in Fig. 2( wherein the three attenuation limit lines are distorted in accordance with the variation in the input voltage of the device 2'! to be tested. In the remaining wave forms of Fig. 2, the attenuation lines remain linear and horizontal as the device 2! being tested is of such high impedance that the slight variation of the input impedance fails to produce any input voltage pulldown large enough to affect the attenuation reference lines.

This circuit also utilizes the varying input impedance as a means for tuning the device 2! to be tested. This feature is particularly useful when the output voltage from the device 2'! does not reach a large enough magnitude to present a useful frequency response curve on the screen of the cathode ray tube 22. For instance, multistage filters require considerable preliminary tuning before any appreciable output voltage isv obtained. Thus by using the input impedance pulldown voltage, it is possible to tune n'iulti-stage filters by adjusting the wave forms of the attenuation lines to the required resonant peaks.

Referring again to Fig. 7, the frequency response voltage from the output jack Gill is supplied to a grid we of the gate tube 353 through an input jack 566 and a shielded or coaxial cable conductor 46?. The attenuation level voltages from the output jacks 628, 450 and 36B are applied to a plurality of grids 168, see and N8 of the gate tubes 354, 365, 355 through a plurality of input jacks use, 481-,- 482 and a plurality of shieldedor coaxial cable conductors 483, 484 and 485. The plurality of gate tubes 3 .53,. 364, 365 and. 386 conduct in the sequence that has previously been described in conjunction with the ring counter system 268 so that during the conduction period of each tube, the voltage impressed 0n the tube grids is amplified and applied to a conductor which is connected to the B supply 232 through a choke coil ll! and a plate load resistor 412. The choke coil d'il provides shunt frequency compensation to prevent any distortion of the wave forms. Upon completion of one sequence of the tube conductions, a composi-tie signal comprising a frequency response voltage and three attenuation voltages is applied to a grid $81 of a detector tube 688 through a condenser 490'. denser 691i is very small in capacitance so that the low frequency components of the voltage produced by the differences in the plate curren of the four tubes will be removed.

The detector tube G88 is biased so that the plate current flows only in response to the positive portion of the applied voltage thus producing a partially detected voltage at the high potential end of a cathode resistor 505. The voltage is further detected by a filter circuit comprising condensers El and 552 and a choke coil 593. The voltage is finally detected by the choke coil 5% which removes the radio frequency component of the applied voltage. This detected voltage is applied across a grid bias resistor 584 and then to a grid 595 of an amplifier tube through a conductor 59?. The detected voltage varies the plate current flowing through a cathode resistor 568 which is connected in common with a cathode 5 39 of an amplifier tube 5H]. A grid 520 of the tube 5!? is connected to ground through a conductor 52I so that the varying voltage developed across the resistor 508 by the varying plate current through the tube 555 servesto vary the plate current flowing through the tube 5H1. The voltage developed at the low potent-ial end of a plate load resistor 522 is connected to an output jack 523 through a coupling condenser 52 2. The tubes 596 and 5H} form a non-inverting amplifier arrangement whereby the detected Wave form comprising the frequency response voltage and the attenuation voltages is amplified and applied to the output jack- 523. From the output jack 523, the voltage is applied through an external connector to the vertical deflection means of the cathode ray tube 22.

From the foregoing detailed descriptions, it is believed that the operation of the circuit will now be understood.

As soon as the A. C. voltage is applied to the plug terminals 23-5 and are, an alternating voltage is applied to the filament transformer 234 and thence to the grid 290 of the synchronizing tube 28?. The pulse produced by this tube is coupled to the multivib-rator 269 so that the pulse The con- 7 14 applied to the grid 40 of the saw-tooth oscillator tube AI: from the multivibrator 259 is in synchronism. both with the 60 cycle line voltage and the switching rate of. the multivibrator 269.

The pulse applied to the grid 48 determines the rate at which the tube 4| fires and, therefore, controls the saw-tooth voltage output from the oscillator 41. This saw-tooth voltage is applied to: the horizontal deflection means of the cathode ray tube 22 through the output jack dB to provide a linear sweeping voltage. The sawtooth wave is also applied to the control grid 8 of the reactance tube 58 to control the flow of plate current therethrough. By varying the flow of the plate current in accordance with the impressed s-aw-tooth'. voltage, the reactance effect of the tube 53 on the tuned circuit of the oscillator tube 66 is also varied in accordance with the impressed saw-tooth wave form. The result of the varying reactance is to vary the frequency of oscillation of the oscillator tube 65 so thatthe output voltage from the tube 66" is frequency modulated in accordance with the original sawtooth voltage.

The frequency modulated voltage from the tube 66 is applied to the control grid 63 of the mixer tube 6b, the control grid 81 of which is connected to a continuous wave oscillator tube 89. The inductance coil 8| of the tuned circuit of the tube is adjusted to the value which produces: a constant frequency output from the tube which, when heterodyned against the frequency modulated voltage from the tube 65, produces a frequency modulated voltage of the desired mean frequency. In this manner, the range of frequencies over which the device 2? is to be tested. may be varied at will. The voltage output from the mixer tube 69 is filtered in the band pass filter H2 and is subsequently amplified and limited in the tubes iil, 534 and i555. This constant amplitude frequency modulated voltage is applied te -the output jack I13.

The frequency modulated voltage from the tube 69 is also applied through a pair of amplifying tubes 98 and 02 to a plurality of crystal filters I06 and: I51. The switching means I23 is then moved to select the crystal H5 which passes a voltage of a desired frequency. The switching means I is actuated to select the crystal I2I which passes a voltage of a second desired frequency. The difference in frequency at which the crystal filters I Ill and EEI pass their respective voltage pulses is the absolute diiference in frequency between the two negative intensity frequency bandwidth markers which are to appear on the screen of the cathode ray tube 22.

The slight voltage disturbances produced in the envelope of the frequency modulated signal applied to the crystals IIS and I2I are shaped into clear negative intensity pulses by the action of the two positive feedback circuits comprising the tubes I-25, I2? and the tubes I 53, I 62. The two pulses from the pulse shapers are mixed together inthe tube It! to provide a composite signal including both frequency markers removed from each other by a distance representative of the difference in frequencies at which the crystals I) and I 21 pass their respective voltages. This voltage is clipped and amplified by the tubes i86 202 and subsequently coupled to the output jack 203. These negative intensity marking pips are supplied from the jack 203- to the intensity modulating means of the cathode ray tube 22 through an external conductor.

The frequency modulated voltage applied to the jack H3 is connected to the jack 384 through an external conductor (not shown). From the input jack 3% the voltage is applied through a variable resistor 333 and a cathode follower tube SSi to output jack 388. The device 2'! which is to be tested is connected to the jack 388 through an external cable to receive the testing voltage and is also connected to the jack 39! through an external cable. The jack 39I applies the frequency response voltage from the device 2"! through the conductor sec to the amplifier tube 393. The amplified voltage from the tube 399 is applied to the output jack WI through the attenuator 332.

A voltage is derived from the input of the device 2? in substantial accordance with the variation of the input impedance of the device 2? and applied through a pair of frequency compensated. amplifiers 48:2, 39? to a plurality of variable resistors $39, 12%, 2 These resistors are then adjusted to compensate for the unequal losses in the tubes M2 and 53. From the resistors 3533, 459, and @213, the Voltage is applied to the attenuators 3533, 38%, and 395 through the impedance matching cathode follower tubes 424 M2 and M3 and then to the output jacks 628, 459 and sec. The attenuators 393, 394, 395 are set to provide the desired attenuation level lines which are to appear on the screen of the cathode ray tube 22.

A plurality of external connectors connect the output jacks MI, 428, 450, and 460 to the input jacks 456, 330, 48I and 482. From the input jacks, the voltage is applied to the grids of the gate tubes 353, 36 2, 365, 365 which are rendered conductive in a predetermined sequence by the ring counter system 268. The plurality of attenuation voltages and the frequency responsevoltage are mixed together by the gate tubes and the resultant composite signal is detected and amplified by the tubes 488, 596 and cm. The final voltage applied to the output jack 523 is a composite clear voltage wave form shown in conjunction with the output jack 523 in Fig. 6, and is applied to the vertical deflection means of the cathode ray tube 22 through external connection means.

Referring now to Fig. 2, (a) shows the plurality of traces formed on the screen of the: cathode ray tube when the device 2'. is not tuned. The upper three lines from bottom to top represent the maximum, minimum and zero ref erence attenuation levels, respectively. The lowermost of the traces is the untuned frequency response trace of the device 21 under test. The two clashes in each of the traces are due to the negative intensity frequency bandwidth markers which are applied to the intensity modulating means of the cathode ray tube 22. The segment of the trace intercepted between the two marker dashes represents the particular desired bandwidth. Inasmuch as the internal impedance of the device 2'5 is not of the same magnitude as the internal impedance of the voltage source, the input voltage pulldown is so small that the attenuation lines are not changed from the straight horizontal trace. Fig. 2(9) shows a set of attenuation lines which are modified in wave form by the input impedance variation.

The plurality of scales provided on the screen of the cathode ray tube 22 are used in making many different tests either singly or concurrently. In Fig. 2(1)) the device 21 is correctly tuned in both the primary and the secondary windings so that the two-peaked response curve is centered in the desired frequency band as indicated by the frequency markers. When it is desired to check the gain characteristic of the device 27, an external amplifier may be provided at the output of the test device 21 so that the output may be raised or lowered through a certain predetermined range. If it is not possible to bring the mid-frequency response of the test curve to the zero reference line by varying the gain of the amplifier through the allowable range then the device 21 will be rejected. Fig. 2(b) shows a curve in which the gain characteristic of the device is too low because the mid-frequency valley is below the zero reference line. In a like manner, Fig. 2(c) shows a frequency response curve in which the gain characteristic is too high because the mid-band response may not be lowered to the zero reference line. Fig. 2(d) shows a typical wave form of a correctly tuned resonant device which has an acceptable gain characteristic.

It is also possible to test the width of the response curve by using the indicia on the screen of the cathode ray tube 22. Fig. 2(e) shows a frequency response curve in which the attenuation characteristic is too narrow because the attenuation of the device 21 is too large within the desired pass band indicated by the frequency markers. Fig. 2(f) shows a frequency response curve in which the attenuation characteristic of the device 21 is too wide inasmuch as the value of attenuation is below the minimum value of attenuation, shown by the reference line, for a range of frequencies larger than the desired frequency band indicated by the frequency markers.

Fig. 2(9) shows a properly tuned resonant device 2'l wherein the input voltage pulldown is sufficient to change the wave forms of the attenuation lines as hereinbefore described. Fig. 2(h) shows a wave form from a device 2! which has a proper gain characteristic and an acceptable band width, however, the mid-frequency of the response curve is left of the center of the desired frequency band. By returning the device 21, it is possible to move the mid-frequency point to coincide with the center of the desired frequency band indicated by the frequency markers.

Figs. 4(ai-c) show wave forms which are typical of various modifications and uses of the invention. Fig. 4(a) shows a wave form in which the curve tracer is used to present the frequency response curve of a frequency discriminator. Fig. 4(b) illustrates a wave form presented by a sharp cut-01f filter while Fig. 4(0) shows the wave form of the frequency response of a transducer.

It is to be understood that the above described arrangements are simply illustrative of the application of the principles of the invention. Numerous other arrangements may be readily devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

What is claimed is:

1. A marking signal generator comprising a source of voltage of cyclically varying frequency, a plurality of resonant devices energized by the variable frequency voltage, said resonant devices being tuned to different frequencies within the range of the varying frequency voltage, a plurality of positive feedback pulse generators, each of said positive feedback pulse generators being energized by one of said resonant devices to produce a marking pulse, and mixing means connected to each of the feedback pulse generators 17 to form a composite marking signal including a plurality of marking pulses separated from each other by the difference in frequency between the resonant frequencies of the resonant devices.

2. A markin signal generator comprising a source of voltage of cyclically varying frequency, a plurality of resonant devices energized by the varying frequency voltage to produce a plurality of voltage disturbances, each of said resonant devices being tuned to a different frequency within the range of the varying frequency voltage so that each of the plurality of voltage disturbances is a different frequency, means connected to each of the resonant devices for clipping and shaping the voltage disturbance to form a marking pip, and mixing means connected to each of the clipping and shaping means to combine the marking pips into a marking signal wherein the pips are separated from each other by the difierences between the resonant frequencies of the resonant devices.

3. A marking signal generator comprising a source of voltage of cyclically varying frequency, a plurality of resonant devices of different resonant frequencies, means for selectively energizing a predetermined number of said resonant devices to produce a predetermined number of voltage disturbances, means connected to said predetermined resonant devices for shaping and clipping the voltage disturbances to produce a predetermined number of marking pips, and mixing means for combining the marking pips into a marking signal wherein the pips are separated from each other by the differences in frequency between the predetermined resonant devices.

4. A marking signal generator comprising means for producing a plurality of voltage disturbances of different predetermined frequencies, a plurality of shaping and clipping means, each of said shaping and clipping means being energized by a different voltage disturbance to produce a marking pip, each of said shaping and clipping means including a positive feedback amplifier with grid voltage clipping, and mixing means energized by a plurality of the marking pips to produce a composite marking signal wherein the pips are separated from each other by distances representative of the difierent predetermined frequencies.

5. A marking signal generator comprising a source of voltage of cyclically varying frequency, a plurality of crystals of different resonant frequencies, means connected to the said voltage source for selectively energizing predetermined crystals to produce a plurality of voltage disturbances at different predetermined frequencies, means energized by the plurality of voltage disturbances for producing a plurality of marking pips, and means for combining the plurality of marking pips into a composite marking signal wherein the pips are separated by distances representative of the corresponding differences between the predetermined frequencies.

6. A marking pulse generator comprising a. means for producing a voltage disturbance, a first amplifier energized by the voltage disturbance to produce an amplified output, a second amplifier having a grid energized by the amplifled output, a first rectifier connected to the grid for removing negative portions of the amplified output, a voltage divider, said second amplifier providing a second amplified output connected to the voltage divider, a second rectifier connected in parallel with the voltage divider to remove positive portions of the second amplified output, and means for applying a portion of the second amplified output from the voltage divider to the first amplifier for reinforcing the original voltage disturbance to thereby produce a sharp marking pip.

7. A marking signal generator comprising means for producing a plurality of voltage disturbances of different predetermined frequencies, a plurality of shaping and clipping means, each of said shaping and clipping means being energized by a different voltage disturbance to produce a marking pip, a common plate mixer tube having cathode load resistors and grids of the tube connected to ground, and means for coupling each of the marking pips to one of the cathode load resistors whereby the mixer tube generates a composite marking signal wherein the pips are separated from each other by distances representative of the diflerent predetermined frequencies.

8. A marking signal generator comprising a source of voltage of cyclically varying fre uency, a plurality of crystal filters, each of said crystal filters being tuned to a different frequency within the range of the varying frequency voltage, means for selectively applying the varying frequency voltage to a desired number of the plurality of crystal filters to produce a number of voltage pulses, means connected to the filters for shaping the voltage pulses to form marking pulses, and mixing means energized by the marking pulses for combining these pulses into a mark- References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,145,332 Bedford Jan. 31, 1939 2,226,459 Bingley Dec. 24, 1940 2,292,100 Bliss Aug. 4, 1942 2,484,352

Miller et al Oct. 11, 1949 

